This invention relates to nuclear magnetic resonance (NMR) imaging techniques. More specifically, this invention relates to the acquisition of a plurality of NMR signals during a single pulse sequence in which the NMR signals are acquired at different effective bandwidths.
The nuclear magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, NMR has been developed as a medical diagnostic modality having applications in imaging the anatomy. As is now well known, the NMR phenomenon can be excited within an object, such as a human patient, by applying a homogeneous polarizing magnetic field, B.sub.0, and by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil is used to detect the NMR signals, frequently in the form of spin echoes, emanating from the patient lying within the field of the RF coil. In the course of a complete NMR scan, a plurality of NMR signals are typically observed. The NMR signals are used to reconstruct an NMR image.
The most common method for producing an image using NMR is to spatially encode the NMR signal using magnetic field gradients. In one such method referred to as "spin warp", for example, a phase encoding gradient field G.sub.y is applied to impart a phase change to the spin which is a function of the position of the spin along the y axis. During the subsequent acquisition of the NMR signal, a second position encoding gradient field G.sub.x (i.e. the "read-out gradient") is applied to continuously change the phase of the NMR signal as it is sampled. The amount of phase change between samples is a function of the location of the spin along the x axis. By performing a series of such measurements, or "views", with different values for the phase encoding gradient G.sub.y, a two-dimensional array of NMR sample data is acquired. An image of the spin density in the x-y plane can be constructed by performing an inverse Fourier transform on this data.
There are three important factors in this NMR method which influence the quality of the reconstructed image: signal-to-noise ratio; aliasing artifacts; and chemical shift artifacts. The signal-to-noise ratio (SNR) of the acquired NMR signals is critical if noise in the image is to be kept to acceptable levels. One method of doing this is to reduce the bandwidth of the encoded NMR signals by using a weaker read-out gradient. Unfortunately, a reduction in the bandwidth of the encoded NMR signal by weakening the read-out gradient increases chemical shift artifacts. The reason for this is that the problem of chemical shift artifacts is inversely related to the read-out gradient strength. Aliasing artifacts result when the NMR signals produced by spins at the spatial extremes of the image undergo a phase shift whose absolute value is greater than 180.degree. between samples. For example, a phase shift in the signal of 185.degree. produced by spins located at the extreme right side of the image is interpreted by the reconstruction process as emanating from spin located on the left side of the image where the phase shift is -175.degree.. As a result, the spin density is misplaced, or "aliased", in the reconstructed image. The aliasing problem in the read-out gradient direction can be significantly reduced by limiting the bandwidth of the acquired NMR signals to those below the Nyquist frequency of the sampling, such that components produced by spins at the extremes are filtered out and do not appear in the reconstructed image.